This error happens because Ubuntu SSO cannot be contacted by ubuntuone-login process. Eventually the latter gives up and throws a DBus error.
If you are experiencing this issue please check whether you have a custom python installation used by default:
which python
Expected output: /usr/bin/python
If you have e.g. another python binary in /usr/local/bin and `which python` shows it then ubuntu-sso-client will fail to start leading to this issue.
If you don't have another python installation please leave a comment with the output of:
/usr/lib/ubuntu-sso-client/ubuntu-sso-login-qt --app_name test